88 240 DL

I am running 87 octane with no problems in any conditions.

Perhaps your knock sensor is bad, you are running an overly lean mixture, radiator may be a bit plugged and engine is staying hotter, or you have carbon buildup in the combustion chamber. Most depends on the age and condition of the car with the exception of the knock sensor and lean condition.

Test the knock sensor and check your spark plugs to determine if they are very white or a normal tan color. If they are "bone white" the fuel mixture is on the lean side.